Norwalk-like viruses (NLV), a genus within the Caliciviridae, are the major cause of epidemic gastroenteritis in the US and a significant cause of severe diarrhea in young children[16]. Based on their low infectious dose, high transmissibility and economic impact, NLVs have been classified as Bioterrorism Category B Priority Pathogens by the U.S. Centers for Disease Control and Prevention and the National Institute of Allergy and Infectious Diseases (http://www.niaid.nih.gov/dmid/bioterrorism/). The proposal is responsive to NIH NOT-AI-02-023, Biodefense and Emerging Infectious Disease Research Opportunities (http:/Iwww.niaid.nih.gov/dmidlbioterrorismlrfalplat.htm). The long-term goals of this proposal are to elucidate the molecular mechanisms governing human susceptibility to NLV infection. In this capacity, we will use human challenge models to test various hypotheses that specific human susceptibility alleles and acquired immune factors determine NLV infection outcomes and pathogenicity. It is anticipated that the identification of host factors and responses that influence NLV pathogenesis will reveal fundamental insights into the molecular biology of these viruses, simultaneously allowing for the development of NLV vaccine and therapeutic strategies. A second goal is to develop NLV vaccines using alphaviruses as heterologous vaccine vectors. Venezuelan equine encephalitis virus (VEE)-based vaccines elicit high levels of mucosal and systemic immunity against NLVs [2, 3, 17-19]. We will test various hypotheses that VEE replicon particles (VRPs) encoding different genogroup I and II (GI and GII) NLV capsid genes elicit strong systemic, cellular and mucosal immune responses against homologous and heterologous NLVs and are superior to the current standards for NLV vaccine development (NLV recombinant virus-like particles (VLPs) and edible vaccines).